The performance, fuel economy, and emissions of an internal combustion engine may be altered by varying the engine valve timing. One method to control valve timing is presented in U.S. Pat. No. 6,904,881. In one embodiment, a method is described wherein the timing adjustment of an intake valve is prohibited until the valve timing of the exhaust valve approaches the control target. In other words, this approach limits intake cam movement based on how well the exhaust cam follows a desired trajectory or profile. The objective of this method is to regulate the amount of overlap between exhaust and intake valves. That is, the approach seeks to control the amount of time that the intake and exhaust valves are simultaneously open. By regulating the amount of overlap, the method intends to improve combustion stability and engine torque while the engine accelerates and the cams are repositioned.
The above-mentioned method can also have several disadvantages. Specifically, the method always limits the movement of an intake cam based on the position of an exhaust cam. Strictly constraining the movement of the intake cam based on the position of the exhaust cam can be undesirable during some conditions. For example, when an exhaust cam is being retarded faster than the rate that an intake cam can be adjusted, it is possible to exceed a desired overlap because retarding the exhaust cam faster than the intake cam will increase the relative distance between the exhaust valve closing and the intake valve opening, thereby increasing the overlap and the possibility of engine misfire. Further, engine power can be reduced when overlap is increased to the point where it is possible to pass an uncombusted air-fuel charge from the intake manifold to the exhaust manifold (blow-through).
The inventors herein have recognized the above-mentioned disadvantages and have developed a method to control engine valves that offers substantial improvements.